1. Field of the Invention
The present invention relates to an optical signal correction apparatus and an optical signal correction method, and particularly to a technique for correcting an amount of tilt of a wavelength multiplexed signal in which a plurality of signals are multiplexed.
2. Description of Related Art
An example of a wavelength division multiplexing transmission apparatus is explained with reference to FIG. 8. FIG. 8 is a view showing an example of the wavelength division multiplexing transmission apparatus. A wavelength division multiplexing transmission apparatus 9 includes optical amplifiers 90 and 92, and an attenuator 91. Note that FIG. 8 is drawn by the inventor of the present invention and not a view of a related arr.
The optical amplifier 90 receives optical wavelength multiplexed signal transmitted from an upstream transmission path. The optical amplifier 90 amplifies the optical wavelength multiplexed signal received from the transmission path by a fixed gain. Specifically, the optical amplifier 90 amplifies each of a plurality of optical main signals included in the optical wavelength multiplexed signal by the fixed gain. For example, when the gain is 10 dB, the optical amplifier 90 amplifies a 0 dBm main signal to 10 dbm, and amplifies a −1 dBm main signal to 9 dBm. The optical amplifier 90 outputs the amplified optical wavelength multiplexed signal to the attenuator 91.
The attenuator 91 corrects a tilt of the optical wavelength multiplexed signal by optically attenuating the optical wavelength multiplexed signal output from the optical amplifier 90. The attenuator 91 corrects the optical wavelength multiplexed signal by a design fixed value to have an appropriate tilt. The tilt here is a slope when regarding characteristics in which the power of optical main signal decreases or increases along with a longer wavelength or shorter wavelength in the optical wavelength multiplexed signal as a line. The attenuator 91 outputs the optical wavelength multiplexed signal with the corrected tilt to the optical amplifier 92.
The optical amplifier 92 amplifies the optical wavelength multiplexed signal output from the attenuator 91 to have fixed power at any time. Specifically, the optical amplifier 92 amplifies the optical wavelength multiplexed signal so that the total power of a plurality of optical main signals included in the wavelength multiplexed signal will always have the fixed power. The optical amplifier 92 transmits the amplified optical wavelength multiplexed signal to a downstream transmission path.
A downstream device cannot successfully recognize the optical main signal when the power of the optical main signal is too large or too small. As described above, the wavelength division multiplex transmission apparatus 9 corrects the optical wavelength multiplexed signal to have an appropriate tilt and amplifies the optical wavelength multiplexed signal to have the fixed power at any time, so that the downstream device can recognize the optical main signal.
However, the wavelength division multiplex transmission apparatus 9 mentioned above corrects the tilt only by the design fixed value. Therefore, when the optical wavelength multiplexed signal has an unexpected tilt, the optical wavelength multiplexed signal cannot be corrected to have an appropriate tilt. Thus, a difference is generated in the power between the optical main signals included in the optical wavelength multiplexed signal. In other words, there has been the optical main signal with too large power and the optical main signal with too small power. Further, as the optical main signal with too small power is more subject to the influence of noise, the signal-to-noise ratio (S/N ratio) will deteriorate. Accordingly, there is a problem in the abovementioned wavelength division multiplex transmission apparatus 9 that the transmission quality of the optical wavelength multiplexed signal deteriorates and the optical wavelength multiplexed signal cannot be successfully transmitted to the downstream device.
Japanese Unexamined Patent Application Publication No. 2003-298529 discloses a technique to calculate an optimal gain compensation amount of a variable gain compensator by estimating a gain tilt amount using a predetermined formula from input monitor information indicating a value which monitored input power of an optical amplifier in each repeater.
Japanese Unexamined Patent Application Publication No. 2003-298529 further discloses a tilt amount monitor which enables calculation of the compensation quantity of the variable gain compensator. FIG. 9 shows a configuration of the tilt amount monitor. The tilt amount monitor includes a coupler 101, a variable gain compensator 102, a control circuit 103, a demultiplexer 104, and monitor PDs 105 and 106. The coupler 101 divides a part of optical signal and outputs it to the demultiplexer 104. The demultiplexer 104 demultiplexes the optical signal output from the coupler 101 into optical signal λ1 on the shorter wavelength side and optical signal λ2 on the longer wavelength side with respect to a point of dynamic gain tilt characteristics. The demultiplexer 104 outputs the optical signals λ1 and λ2 obtained by the demultiplex respectively to the monitor PDs 105 and 106. The monitor PDs 105 and 106 convert each of the optical signals λ1 and λ2 into signal power information P1 and P2, which is proportional to the signal power, and output the signal power information P1 and P2 to the control circuit 103. The control circuit 103 compares the signal power information P1 and the signal power information P2, and calculates a code of a slope of the tilt and an absolute amount of the of slope of tilt. Then, the control circuit 103 calculates the compensation amount of the variable gain compensator.
However, Japanese Unexamined Patent Application Publication No. 2003-298529 does not disclose a technique that, when the operation status is changed and the number of optical signals multiplexed on the optical wavelength multiplexed signal is changed, calculates the tilt according to the changed number of optical signals. Specifically, the technique disclosed in Japanese Unexamined Patent Application Publication No. 2003-298529 cannot calculate a correct tilt when the number of optical signals included in the optical wavelength multiplexed signal is changed. Therefore, the present inventor has found a problem that an appropriate compensation amount cannot be calculated and the transmission quality of the optical wavelength multiplexed signal cannot be improved.
In the technique explained above, there is a problem that when the number of the optical signals multiplexed on the optical wavelength multiplexed signal is changed, the transmission quality of the optical wavelength multiplexed signal cannot be improved.